1. Field of the Invention
This invention generally relates to those devices used to dampen vibratory motion, and/or adjust sag, in overhead lines (e.g., electrical power transmission lines, telephone lines, ground wires, guy wires and the like). These vibration damping and sag adjustment functions are both accomplished by virtue of the fact that the highly concentrated body weight of these devices is placed at select locations on such lines.
The two most common vibratory motions dampened by such devices are so-called "aeolian" vibrations, and so-called "galloping" vibrations. Aeolian vibrations are caused by wind eddies created on the lee side of overhead lines during relatively light winds (e.g., those having velocities of about 1 to 15 miles per hour). If the frequency of such eddies happens to coincide with one of the many natural frequencies of a given line span, the resulting vibratory forces can weaken or damage the line at its insulator or anchor points.
Galloping vibrations are often caused by formation of ice on the windward side of an overhead line. In effect, ice formation of this kind gives the line an airfoil-like cross sectional configuration. Hence, even under relatively moderate winds (e.g., those having velocities of about 10-35 miles per hour), these airfoil-like configurations can create very powerful aerodynamic lift and drag forces on these lines. These forces sometimes induce "galloping" motions in such lines. These galloping motions are often exceedingly destructive to the lines, their insulator and anchor points and even the poles, towers, etc. that support them.
Vibration damping devices also are used to reduce sags in, and/or increase local clearances of, a given overhead line span. For example, a line span having a catenary sag contour (which is the natural curve that results from the uniform weight per unit of length of a flexible line) can be adjusted such that a given section of that line can be raised while another section is lowered. This type of sag adjustment is accomplished by weighting a given line span at one or more select points.
Those skilled in this art also will appreciate that the magnitude of a given line sag also may be affected by such factors as: the weight of the line per lineal foot, its diameter, the tension under which it is installed, the length of the span under consideration and the insulator and other hardware configurations employed on it. Similarly, because the devices of this patent disclosure provide both their vibration damping function and their sag adjustment function by virtue of their creation of heavy, concentrated weight(s) at select points along a given line span, the overall effects of the weight of these devices on a given line span also must be taken into account. For example, the effects of their concentrated weight (s), with respect to increased vertical loads, differential longitudinal loads and increased tensions on the conductors, cables, strands, wires, insulators, hardware, dampers and support structures, all must be taken into consideration. The effects of temperature, wind and ice conditions that a given overhead line will encounter also must be considered when considering the deployment of vibration damping/sag adjustment devices.
Such devices also are sometimes used to reduce so-called radio influence voltage (RIV) problems that can result from inadequate loading of insulator strings that are frequently placed on such lines.
Such devices are also useful in counteracting so-called "uplift" conditions (static uplifting forces that result from constructing overhead lines on the lower regions of step terrains), especially when the line support structures being used in such lower regions are not provided with bracing to combat uplift forces. Such devices are also sometimes used in forming so-called "jumper strings" at line end ("dead end") support structures.
2. Description of Prior Art
U.S. Pat. No. 1,676,006 discloses an anti-oscillation scheme for an overhead line such as an electrical power line. The scheme employs weighted devices at node points in the overhead wire system such that the periods of oscillation of each adjacent line span are rendered incommensurable. These anti-oscillation devices are in the form of a ball-like clamp that is split in the middle to form two hemisphere-like portions. The overhead line passes through a hole in the center of the ball-like clamp. The two hemisphere-like portions of the clamp are held together by clamping bolts. The hole through which the line passes is substantially circular (i.e., each hemisphere of the clamp has a uniform semicircular trough passing through its center).
U.S. Pat. No. 3,553,345 discloses a overhead line vibration damping device comprised of a tube that is attached to a ball-like weighting device. The ball-like weighting device is comprised of two hemispherical portions that are secured together with bolts. The ball-like clamp is adapted to fit around the overhead line and one end of the tube. The ball-like device, in effect, has a two sized hole, i.e., a hole having two different diameters. The first part of the hole encompasses the cable and the second part encompasses one end of the tube. The cable itself passes through both the ball-like device and the tube.
U.S. Pat. No. 4,686,325 teaches a method for adjusting the catenary sag in cable systems. The method essentially involves loading adjacent spans in order to "take up" certain local sag conditions in the overall line system.
U.S. Pat. No. 2,831,048 teaches a device for damping "galloping" in overhead lines. The device comprises a (1) lever arm that is pivotally mounted on a cross arm of a support, (2) an insulator connected to one end of the lever (the insulator supports the line), (3) a weight that is supported adjacent to the other end of the lever arm (in order to counterbalance the mass of the insulator and line) and (4) a restraining member connected to the cross arm which all, acting in concert, serve to dampen gallop in an overhead line.
The prior art also discloses that a wide variety of materials have been used to make vibration damping/sag adjustment devices. For example, lead (and lead alloys), zinc (and zinc alloys), specific lead/zinc alloys, tungsten (and tungsten alloys), cast iron, polymeric materials (such as so-called "hard rubber") and even concrete have been used to make vibration damping/sag adjustment devices.